Recently, display devices have been rapidly developed and among them, liquid crystal display devices may be representative display devices currently used.
A liquid crystal display device (LCD) is a display device that expresses pixels by using the principle of selectively penetrating light depending on the changes in polarization according to the arrangement of liquid crystal disposed between polarizing plates. However, when the display device is composed of only the liquid crystal and polarizing plates, there are problems in that brightness or contrast significantly deteriorates, light leakage occurs, and the like when a screen is observed not at the front surface thereof that is vertical to the screen but at different angles that deviate from the vertical.
In order to solve these problems, an optical compensation film such as a retardation film or a viewing angle compensation film is included in the polarizing plate along with a polarizer, or the retardation film, the viewing angle compensation film and the like are separately adhered to a display panel and then used. When an optical film such as a retardation film, a viewing angle compensation film and the like is inserted between a polarizing plate and a liquid crystal display element as described above, the color change of a liquid crystal display device (LCD) may be reduced and the viewing angle may be widened, thereby improving luminance.
The optical film in order to be used for the above-described use is mainly divided into a stretched film prepared by stretching a polymer film to give optical anisotropy and a liquid crystal film prepared by coating a polymeric liquid crystal compound on a plastic film substrate, drying the polymeric liquid crystal compound, and irradiating UV light thereon to cure the polymeric liquid crystal compound. The stretched film and liquid crystal film used as optical films have optical anisotropy, and particularly, the liquid crystal film may have various types of optical properties, which are difficult to be implemented by the stretched film.
That is, liquid crystal may be divided into a disc-type liquid crystal and a rod-type liquid crystal, depending on the shape of the liquid crystal molecules, and among them, the rod-type liquid crystal has various alignment forms such as planar, homeotropic, tilted, splay, cholesteric shapes and the like, and thus optical properties resulting from the various forms are also diverse and unique. Accordingly, various alignment characteristics of liquid crystal may be applied as they are by directly coating a polymeric liquid crystal compound on, for example, an acetyl cellulose substrate used as an optical film to use the acetyl cellulose substrate coated with the polymeric liquid crystal compound as an optical film, thereby obtaining an effect that is not implemented with only a protective film and a stretched film of a polarizer in polarizing plate, and serving as a role of an optical compensation film.
The above-described liquid crystal film (optical anisotropic film) is generally prepared by a method of coating an alignment film composition for forming a liquid crystal alignment film on a plastic substrate, drying and curing the composition to form an alignment film, rubbing the alignment film to give an alignment property, and then coating a polymeric liquid crystal compound thereon, drying and curing the polymeric liquid crystal compound to fix the compound. However, in the case of a method of forming an alignment film by the rubbing type as described above, there are difficulties in the preparation process due to drawbacks in the process. That is, in the alignment technology by the rubbing type in the related art, which is a kind of contact type, there are impurities and surface defects, and a washing process for removing impurities and surface defects is needed, thereby causing a problem in that the process is complicated and costs a lot.
In order to solve the problem, a technology in which an alignment property is intended to be given to an alignment film by using light has been developed as an attempt of a non-contact type. That is, in order to complement drawbacks of the rubbing process, a light alignment film that gives alignment characteristics to the alignment film in the non-contact type by using light have been currently developed to achieve process simplification, cost reduction and the like. For this purpose, there is need for a light controlling functional material that may control the changes in optical, chemical and physical characteristics caused by generating a structural change corresponding to light energy with various specific wavelengths.